1. Field of the Invention
The present invention relates to a head position control device and medium storage device which controls an eccentricity of a head according to an eccentricity of a disk, and more particularly to a head position control device and a medium storage device for preventing a step difference of drive current which is generated when one head is switched to another head.
2. Description of the Related Art
In medium storage device, such as a magnetic disk device and optical disk device, a plurality of heads are positioned by a same actuator so as to face different disk surfaces. For example, a head is positioned onto a target track by performing seek control, which moves the head to a target track of a facing disk, and performing following control, which is performed thereafter.
As FIG. 24 shows, in this disk device, eccentricity of a disk exists. For example, a locus 110 of a servo signal on one disk surface facing a head A and a locus 112 of a servo signal on another disk surface facing a head B are eccentric with respect to the rotation center of a spindle motor which rotates the disk.
A position is detected by reading a servo signal, and by controlling eccentricity based on the position so as to follow up the servo signal, a position of a head is controlled along the servo signal of a disk which the head is facing, therefore such eccentricity does not become a problem between one head and a disk that the head is facing.
On the other hand, as FIG. 24 shows, loci 110 and 112 of servo signals on two disk surfaces with respect to the rotation center of the motor of the disk device may be different from each other, that is, the eccentricity of a head A and the eccentricity of a head B may be different in some cases. In the case of a system of performing the recording of servo signals to an individual disk before assembling the device, and assembling the disk in the device (called media STW: Servo Track Write), an eccentricity is generated and the difference of eccentricity between disks is large. Also in the case of using a magnetic transfer disk which records servo signals on a disk by magnetic transfer, or using a patterned media where a magnetic recording layer is formed only on the recording tracks of the disk substrate, eccentricity differs between the front and back of the disk.
Because of the difference in eccentricity among disk surfaces, following up the eccentricity of a switched head is required when a head is switched. In other words, if a head A, which is following up the locus 110 of a servo signal, is switched to a head B, the head B must follow up the locus 112 of the servo signal.
FIG. 26 shows an example when the eccentricity locus is different between head A and head B, and shows the changes of current, velocity and position when the head A, which is being position-controlled on a track (locus 110 in FIG. 24), is switched to a head B on a different disk surface. To simplify description, in FIG. 26, it is assumed that the eccentricity of the disk surface of the head A is “0”, and only the disk surface of the head B is eccentric, and the drive current, velocity and position of the head B are shown.
As FIG. 26 shows, the eccentricity correction current of the head B is shown by a sine wave, of which frequency is the same as the rotation frequency. As FIG. 26 shows, immediately after the head A is switched to the head B, the eccentricity correction current is switched from the eccentricity correction of the disk surface of the head A to the eccentricity correction of the disk surface of the head B. By this, a step difference of which value is u0 is generated in a relative current U. The head B has an initial velocity of relative velocity V0 because of the difference of the sine wave loci of the head A and head B. The relative position also changes X0.
However, it has been assumed that the initial velocity when the heads are switched is “0” or the same velocity as the previous head. In head switching, normally the seek control of the new head is performed after switching. Since the initial velocity when the seek control starts is not “0”, oscillation occurs when the head reaches the target position, and it takes time to converge the oscillation, which causes a delay in the seek time.
If current is sharply switched when the heads are switched and the step difference u0 of the eccentricity correction current is large, the actuator resonates and oscillation is generated. Therefore if a sharp step difference is generated in the initial current u0, as shown in FIG. 26, oscillation is generated, and seek time delays.
As a method for lessening the current step difference when the heads are switched, a control system, to which an initial current and current step difference cancellation function are added, shown in FIG. 25, has been proposed (e.g. see Japanese Patent Application Laid-Open No. 2004-022133 (FIG. 5 to FIG. 7)).
As FIG. 25 shows, the servo control system having the eccentricity correction function has: a computing unit 140 which calculates a position error (r−y) between a target position “r” and a current position “y”, a controller C which calculates current to eliminate the position error and outputs it to an actuator P that is a plant, a table 144 which stores an eccentricity correction current for each head so as to follow up an eccentricity of a disk; and an adder 142 which adds an eccentricity correction current corresponding to a selected head (disk surface) Head in the table 144, to a command current from the controller C, and supplies the result to the plant P.
A correction locus generation section 160 determines an initial velocity and current step difference when heads are switched, from the head HeadOld before switching and the head Head after switching, and generates a correction position locus and correction current. The adder 162 adds the current correction locus to the eccentricity correction current from the eccentricity correction table 144 determined from the head Head after switching, so as to cancel the step difference generated by the switching. The computing unit 164 subtracts the position correction locus from the current position “y”, and outputs the result to the computing unit 140.
In other words, the correction locus generation section 160 provides a locus to make the initial velocity zero to the controller C from the outside. The correction locus generation circuit 160 also calculates a locus, including current to cancel the current step difference for the amount of the initial current u0, and corrects the eccentricity correction current.
This conventional proposal is effective when seek control is not performed for the switched head immediately after switching heads. However, if seek control is immediately performed for the switched head, the relationship with seek current values must be considered. In other words, during seeking, the saturation of current and the high frequency components to be added must be considered.
In order to correct velocity so that the velocity is suppressed and resonance is not generated, a current waveform, which is still smooth even after being combined with the seek current, must be added. But in the prior art, as compensation currents in proportion to the initial velocity and current step difference is individually added to the seek current, the waveform of the combined current is changed to vary depending on the velocity and the value of the step difference current. Therefore if these compensation currents are added to the ordinary seek current waveform shown in FIG. 27, the initial current value in seeking suddenly increases, as shown in FIG. 28, or the initial current value in seeking suddenly decreases, as shown in FIG. 29. So, current waveform in initial time of seeking is distorted.
These distortions easily generate noise if there are many high frequency components. For example, if a current with many high frequency components, such as several kHz, is supplied, an user perceives noisy.
In order to decrease the noise, it is necessary to make the waveform shape uniform, and not to include high frequency components. The seek waveform, however, changes depending on the seek time (distance), so according to the prior art, a smooth current waveform is simulated offline individually for the initial velocity and for the current step difference correction so that velocity is suppressed and resonance is not generated, and a number of waveforms corresponding to the seek time must be prepared.
This design takes time, and storing many waveforms according to the change of seek time increases the memory capacity of the device, which impedes cost reduction.
Recently as the use of disk devices expands, disk devices are being installed in acoustic equipment, so designing for silence is demanded for devices, and minimizing operation sounds is required. For this, it is demanded to easily solve the problem of the generation of audible sounds due to the current step difference.